1. Field of the Invention
The present invention relates to a reflective triplet optical form with an external rear aperture stop for cold shielding.
2. Description of the Related Art
Electromagnetic radiation imaging systems have been widely used to image objects at long distances by telescope in the visual and infrared portions of the radiation spectrum.
Generally, optical elements of long distance telescopes may be refractive or reflective. Refractive optical elements are generally effective in controlling and/or preventing aberrations, and may be used in a variety of applications. However, refractive optical elements are not optimum for applications that require a large aperture. Such refractive optical elements (e.g., lenses) are difficult to manufacture and result in greater expense, because it is necessary to manufacture large lens elements for these systems. The large lens elements also tend to flex, resulting in a decrease of image quality. Moreover, refractive optical elements by their nature may also absorb radiation because they are not completely transparent at certain wavelengths.
Reflecting optical elements, on the other hand, have been used in place of refractive optical elements to provide large aperture optical systems. Such reflecting optical elements may be manufactured with greater thickness so as not to flex as easily. In addition, reflective optical elements do not allow light to pass through, but rather, reflect radiation at most wavelengths, resulting in less loss of radiation.
The minimum number of optical elements is generally recognized to be three, to provide the minimum number of parameters that are necessary to correct for and/or prevent spherical aberration, coma, astigmatism and field curvature. An optical imaging system composed of three optical elements is known as a triplet.
Reflective optical triplets are generally constructed such that radiation enters the system from a distant object, is received on a primary mirror, is reflected onto a secondary mirror, is received on a tertiary mirror, and finally, is focused on an image plane where an image of the distant object is formed. Historically, a positive/negative/positive reflective optical triplet can be traced back to the work of Maurice Paul, in 1935 and James G. Baker, in 1945.
Modernly, many prior art reflective optical triplets are composed such that all of the optical elements lie on the optical axis of the optical system. This arrangement results in the occlusion of a significant portion of the radiation entering the system from a distant object, a restriction of the field of view of the system and a constraint on the power distribution between optical elements.
Accordingly, other modern prior art reflective optical triplets are composed such that the field of view is not along the optical axis of the optical system but entirely to one side of it. In the prior art, U.S. Pat. No. 4,240,707 to W. Wetherell and D. Wemble (the “Wetherell '707 patent”, which is incorporated herein by reference, is representative of such a reflective optical triplet. The reflective optical triplet described in the Wetherell '707 patent has an aperture stop on the optical axis and is physically located on the secondary mirror. The entrance pupil to the optical system described in the Wetherell '707 patent is located a large distance behind the optical system, and as such, is virtual.
In addition, yet other modern prior art reflective optical triplets are composed of off-axis portions of the optical elements, such as in the Wetherell patent, but also incorporates a real entrance pupil, such as in U.S. Pat. No. 4,733,955 to L. Cook (the “Cook '955 patent”), which is incorporated herein by reference. The reflective optical triplet in the Cook '955 patent, as a natural result of the real entrance pupil, has a defining front aperture stop coincident with the real entrance pupil.